Cryptosporidiosis is an enteric infection caused by several species of Cryptosporidium parasites. In developing nations, the infection is one of the most common causes of persistent infant diarrhea. No effective drugs or vaccines are available to control cryptosporidiosis. We have found that the composition of the intestinal bacterial microbiota (microbiome) changes in response to C. parvum infection. We have also observed the opposite effect, which suggests that a perturbed (dysbiotic) enteric microbiota can favor parasite proliferation. This proposal focuses on unexplored interactions between Cryptosporidium parvum and the intestinal ecosystem. Specifically, we will investigate to what extent the perturbation of the intestinal microbiota with different antibiotics affects proliferation of C. parvum. In analogy to the increased susceptibility of patients treated long-term with antibiotics to Clostridium difficile, we hypothesize that the severity of cryptosporidiosis is impacted by the relative abundance of certain bacterial taxa or microbial communities in the gut. This hypothesis will be tested with two Specific Aims: Aim 1. The native gut microbiota of mice will be perturbed with a diverse set of antibiotics which do not inhibit C. parvum directly and are not toxic to mice. The proliferation of C. parvum in mice treated with different antibiotics will be measured and 16S amplicon sequencing will be used to quantify the relative abundance of bacterial taxa in the gut. This approach will identify bacterial taxa that positively or negatively correlate in abundance with severity of cryptosporidiosis across all antibiotic treatments. Aim 2. We will investigate whether there is a causal relationship between the abundance of certain bacterial taxa identified in Aim 1 and C. parvum proliferation. To this aim we will transplant dysbiotic fecal microbiota into germ-free mice. Transplant recipients will be infected with C. parvum and the severity of the infection quantified. Transplant and passive transfer experiments will enable us to assess whether any change in the severity of cryptosporidiosis is directly mediated by the intestinal ecosystem. Significance and future research: This research will advance our understanding of the interaction between the gut microbiota and Cryptosporidium parasites. The long-term goal is to test the feasibility of controlling cryptosporidiosis by manipulating the intestinal microbiota using pro- and prebiotics. By identifying bacterial taxa and bacterial communities which influence parasite proliferation, the proposed research will enable future work on the development of low-cost pre- and probiotics to mitigate persistent infection with Cryptosporidium parasites. This approach is novel and is important to pursue because so far traditional drug and vaccine development strategies have failed to identify effective treatments and vaccines against cryptosporidiosis.